A pager is an electronic device used to contact people. It pre-dates mobile phone technology, but similarly uses radio transmissions to communicate between a control/call centre and the recipient.
Early pagers only provided an audio notification, a bleep or series of bleeps, to indicate that you had been paged. The recipient then had to telephone the control/call centre to collect the message.
Later pagers used text messages to provide the recipient with more information.
Pagers are now going out of fashion due to the widescale availability of mobile phone and SMS based text messaging.
Many pagers use the POCSAG on-air protocol.